Honeycomb structures comprising ceramics are used for collecting dusts and other particulate materials contained in exhaust gases discharged from automobiles and exhaust gases generated upon incineration of waste products, for purification of the exhaust gases by removing NOx, CO, HC, etc. using supported catalysts or for purification of high-temperature exhaust gases discharged from production steps in various manufacturing and mining industries. The honeycomb structures are structures formed for collecting and removing particulate materials contained in dust-containing fluids. They have many fluid flowing cells which are partitioned with partition walls and which pierce through the honeycomb structures in axial direction, and the partition walls of the fluid flowing cells have filtering ability, and one end part of a certain fluid flowing cells is sealed and another end part of other fluid flowing cells is sealed. Furthermore, NOx, CO, HC, etc. in exhaust gases can be removed by supporting catalysts on the surface and/or inside of the partition walls of the fluid flowing cells.
Recently, honeycomb structures are used suitably for purification of exhaust gases of diesel engine cars. For purification of exhaust gases discharged from diesel engines, it is necessary to adsorb and remove NOx, CO, HC, etc., and simultaneously to collect particulates discharged from engines, and hence honeycomb structures are used as filters for removal of particulates having catalyst supported therein (DPF: diesel particulate filters). Moreover, honeycomb structures, of which end face is not sealed, are also used as catalyst carriers in diesel engine cars. The diesel engines are mounted on large-sized vehicles such as trucks and buses which are great in cylinder volume and hence the honeycomb structures must also be large-sized.
However, if honeycomb structures are large-sized, mechanical strength thereof decreases and therefore distortions and damages are apt to occur during use, which necessitates employment of a reinforcing means. As the reinforcing means, there is a method of disposing an outer wall on the outer peripheral surface of the honeycomb structures, but this method cannot be applied to large-sized honeycomb structures. For example, there is disclosed a method of providing an outer peripheral wall on the honeycomb structures, followed by firing, but this method gives no consideration to the difference between the proportion of dimensional shrinkage of the outer peripheral wall before and after firing (firing shrinkage percentage) and the proportion of volumetric shrinkage of the inner cell structure (ceramic honeycomb body), and, hence, when the cell structure is made larger in size, distortion is apt to occur during firing due to the difference in proportion of dimensional shrinkage before and after firing, resulting in breakage of the structure (e.g., UM (Utility Model)-B-7-183).
Furthermore, there is disclosed a method of providing an outer peripheral wall on a honeycomb structure to improve mechanical strength (e.g., Japanese Patent No.2604876). However, according to this method, the outer wall is low in strength, and may crack during handling or at the step of supporting of catalysts, which may lead to breaking. Moreover, if the amount of the bonding material such as colloidal silica is increased in order to enhance the strength, the coefficient of thermal expansion of the outer peripheral coat becomes higher than that of the inner cell structure (cordierite honeycomb), and there is the possibility of falling off of the outer peripheral coat part when subjected to thermal cycling. Further, there is disclosed a ceramic honeycomb structure, the outer periphery of which is coated with cordierite particles and/or ceramic fibers, and colloidal silica or colloidal alumina present between the outer periphery and the coat of cordierite particles and/or ceramic fibers (e.g., Japanese Patent No.2613729). However, this ceramic honeycomb structure is low in strength of the outer wall, and the outer wall may crack during handling or at the step of supporting of catalysts, resulting in breakage of the honeycomb structure. Moreover, if the amount of the bonding material such as colloidal silica is increased in order to enhance the strength, the coefficient of thermal expansion of the outer peripheral coat becomes higher than that of the inner cell structure (cordierite honeycomb), and there is the possibility of falling off of the outer peripheral coat part when subjected to thermal cycling.
The present invention has been made in an attempt to solve the above problems, and the object of the present invention is to provide a method for producing a honeycomb structure provided with an outer wall by disposing a material for forming outer wall on the outer peripheral surface, according to which the outer wall hardly cracks during firing and a high-strength and large-sized honeycomb structure can be produced, and, further object is to provide a high-strength and large-sized honeycomb structure with substantially no cracks in the outer wall.